Power division in electromechanical systems



D. GRIMES Feb. 25, 1941.

POWER DIVISIQN IN ELECTROMECHANICL SYSTEMSv Filed Aug. 19, 1959 Patented Feb. 25, 1941r UNITED sT-ATEs PATENT OFFICE David Grimes, Beverly,

assignments, to Philo N. J., assignor, by mesne o Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application August 1.9,

, In Great Britain 1939, Serial No. 291,093

August 26, 1938 y/ 4/ This invention relates to electromechanical /systems and power considerations therein, and

* has for its general object to provide an optimum division of power between various sections of such a system in order that the maximum efliciency or output power may be derived therefrom with a minimum of expense.

The invention applies particularly to electrodynamic sound reproducing apparatus wherein a vibratory or to-and-fro motion of a moving system is obtained by means of electric and/or magnetic forces existing between a conductor on the moving system and a stationary eld structure, and wherein the moving system is supplied with energy from one or more power output tubes, preferably operating as Class A amplifiers.

One of the well known applications of such a system is to be found in audio amplifiers and loudspeakers as used in public address systems, radio receiving sets, and the like, where a large portion of thetotal power available andsupplied by a power Vsupply unit is divided between two irnportant loads, namely, the loudspeaker eld winding and the power output tubes. Now the total power available is usually restricted due to economic and practical considerations, and it is, therefore, of importance, as disclosed herein,that a proper balance ci power consumption be arrived at for these loads. fi

Accordingly, by the present invention there is provided in an electromechanical system, an electrodynamic loudspeaker having a magnetic field structure, a iield winding, and a moving system; vacuum tube power ampliiier means for supplying said moving system with energy whereby it is caused to move relative to said el'd structure; and a power supply means for supplying power to saidveld winding and .to the plate circuit of said amplier means, characterizedin that the power supplied to said plate circuit is comparable to the power supplied to said field winding.

Returning to the illustration of the audio amplier and loudspeaker; and assuming linearity throughout, it has been found experimentally that where the total available power is limited, maximum output power may be derived from the sysvtem when the power supplied to the loudspeaker field is equal to the power supplied to the tube or tubes in the power output stage, and this is true no matter what the relative eiiiciencies of the loudspeaker-or output tubes may be. Whenthe power division between speaker eld and outputV stage is unequal, aloss in acoustic output power is found to result, and this loss increases more rapidly as the degree of mismatch is increased.

Thus while small mismatches may not result in serious losses in acoustic power output, afour-toone mismatch may result ina 36% loss, while an eight-to-one mismatch may resultin a 60% loss.

The same conclusions may be arrived at analytically. Thus assuming that the magneticcircuit of the loudspeaker field structure is operating. below saturation, i. e., 'that the flux varies substantially linearly with field current, it may be shown that if the power W: supplied to the speaker eld be the only variable, then the acoustic power We will vary directly with Wr. Similarly, if the power output tubes be operated on a linear portion of their characteristicit may further be shown that Wa will likewise vary linearly with the power We supplied to the plate circuit of the output stage; or it may be stated that the acoustic power Wa is directly vproportional to the product WfXWo. Assuming then, as is normally the case, that the total power, yWf-i-Wu, availableto be divided between eld and output stage, is

limited, then the product WfXWo will be the maximum when vWf=Wo. Thus by .dividing a limited source of power equally between the loudspeaker eld winding'and the plate circuit of the power output tube or tubes, the maximum possible acoustic power output may be obtained. Conversely, assuming that a specied'acous'tic power output is lrequired, this may be achieved with maximum economy in the design of the power supply unit when the power is divided as hereinbefore disclosed. It has been found that where mismatches of.v 5 to l or more have been made (and even more serious mismatchings are not at all uncommon) the practice of the present invention will make possible either a considerable saving in the cost of the power supply circuit, or, conversely, will considerably increase the acoustic power output which the speaker can deliver.

It is, therefore, an important object of this invention so to utilize a limited power supply source, such as is employed in conjunction with a radio receiver or the like, as to derive the maximum sound output level from' thesound system of said receiver.

Another object of this invention is to disclose the optimum ratio of the powers supplied to the anode circuit'of a Class A output stage and to the field windings of a moving coil loudspeaker.

The invention, as well as other objects thereof, may best be understood by reference to the following description and to the accompanying drawing, in whichi Fig. 1 is a schematic diagram representative of one embodiment of the invention;

Fig. 2 isa schematic diagram of an alternative embodiment of the invention; and

Fig. 3 is a curve showing the relation between acoustic power output and the ratio Wo/Wr.

In Fig. 1 there is schematically shown a portion of a radio receiver which may comprise the usual carrier frequency amplifier and detector circuits audio output stage employing a vacuum tube Vs. an output transformer 5, a loudspeaker L. S. having a moving coil i, and a power supply source which comprises the power transformer 2, rectifier tube V4, and a filter system which may in` clude the series choke F, and the shunt capacitors 3 and 4. v

Since the details of the circuits employed in the amplifier-detector portion of the receiver I, are

of no special interest as regards this invention,

they have been indicated merely by the Itriodes V1 and V2, and by the dotted plate circuit representations 1. In the interests of simplicity, there is shown a single pentode power output tube Va. However, if desired, two such pentodes, or'alternatively, a pair of triodes, may be employed in a push-pull output arrangement. l

With reference to the power supply system, the choke 'F may serve both as the eld winding and ileld structure for the loudspeaker L. S., and as a series-connected choke coil in the filter circuit. The design and construction of electrodynamic loudspeakers of this general type, inV which a voice coil is affixed yto a sound radiator or diaphragm, and arranged to move in a magnetic eld set up by the speaker field F, is well known and requires no further description. The-anode circuits of the tubes V1 and Vzimay be supplied from either, the input or output side of the choke F, but preferably from the latter. According to the invention, the winding comprising the field winding F is so designed that the power expended in said winding is comparable to the power supplied to the output tube Vs. Conversely, it is possible to adopt a. given field winding F and to choose an output tube or tubes Va such that the above-mentioned relation is again obtained. The

power dissipated in the eld winding F depends in general upon the size and kind of wire adopted, upon the number of turns, and upon the mean length of wire per turn.

Fig. 2 represents a modification of the circuits of Fig. 1. Corresponding circuit elements, however, have been given like designations in both figures. In Fig. 2 there is provided a two-section iilter, the first section of whichv includes the series choke coil C, and the second section of which includes the winding F of the speaker field. Three electrolytic capacitors 3, 4 and l0 may be provided as indicated. It will be observed that, with respect to the plate' circuits of the tubes V1 and V2 which are connected to the point B+, the windingsC and F are serially connected between the cathode of the rectifier V4 and the leads 1. However, with respect to the output stage, it may be observed Vthat the eld winding F (serially connected with the resistor 9) is connected in shunt with the plate circuit of the tube Va. This results from the fact that the plate voltage for Va is derived from the point B++ between the windings C and F. If desired, a tapped resistor 8 may be included as shownto provide a bias voltage supply for uses'which ,are well understood in the art. In the embodiment of Fig. 2, the combination of the field winding F and the resistor 9 should be so designed as to provide the desired voltage B+ for the tubes V1 and Vz while simultaneously providing the desired division of power between the field winding F and the plate circuit of the tube Va.

In either of the illustrated embodiments, the

final output stage preferably employs tubes operi.;

ating as Class A amplifiers. Under these conditions, the balance of power between the outstantially fixed, and independent of the amplitude of signals within the rated capacity of the output stage. Where Class B amplifiers are employed, the desired balance of power could in general be achieved only at a particular audio signal level. preferably at a high signal level.

Reference is now directed to Fig. 3 in which relative acoustic output power, W, is shown as a function of the ratio Wo/Wx, i.y e. the ratio of the power supplied to the plate circuit of the output stage to the power supplied to the loudspeaker iield winding. The total power Wa-i-W/ is held constant. It will be observed that maxi mum acoustic output is obtained when the ratio Wo/W: is unity. Similarly it may be seen that the loss in relative acoustic output, as the ratio Wo/Wi departs from unity, is the same for either the said ratio or for its inverse. Thus, for ex-y ample, acoustic output falls to about 64% of its maximum value at ratios of both 4 and V4. For this reason the curve, as plotted, is symmetrical about unity, i. e. about the ratio 1 to 1. It will be seen from the curve that a mismatch of 21/2 to 1 results in only about 18% power loss.

The manner in which the present invention may be practiced to yield a substantial saving in power supply costs may be seen from the following illustration. Assume that it is desired to provide a certain maximum sound output level, and that this can be obtained, in accordance with the prior art, by means of a 3 watt speaker ileld and a power output stage Whose plate circuit is supplied with 12 watts of power; that is, a total of l5 watts for the speaker field and output stage plate circuit. In accordance with the present invention, thesame sound output may be obtained by supplying 6 watts to the speaker eld and 6 watts to the output stage, the product of 3 and 12 being equal to 6 squared, or 36. Thus, in each case the product Wf-i-Wa is equal to 36, and hence the acoustic outputs of the two systems are alike. In the latter case, however, only a totalbf 12 watts need be supplied to the speaker field and output stage, as compared to the 15 watts required in the former case. In the mass production of radio receivers, this difference represents a substantial saving in the cost of power supply and associated circuits.

Similarly, in the example cited, the output stage of the balanced system may employ a tube u or tubes of only one-half the power rating of vthose required inthe unbalanced system of the prior art, in this instance 6 watts as compared totaught by this invention may be secured by means of various circuit arrangements other than those illustrated in the accompanying drawing. which. have been included merely as examples for the purpose of disclosure. Similarly the invention is not to be limited to equal or highly precise di visions oi' power between the loudspeaker ileld winding and the plate circuit of the output tube or tubes. For example, it is deemed that mis matches of the order of 2 to 1, or 1 to 2 fall well within the sphere of this invention. Where such mismatches obtain, it is deemed that the power supplied to the loudspeaker field winding and that supplied to the plate circuit of the output tube or tubes are comparable one to the other, particularly when considered withreference to the relatively great mismatches hereinbefore mentioned.

I claim:

1. In an audio frequency output system, an

lli)

electrodynamic loudspeaker having a eld structure, a field winding wound on said eld structure to provide a magnetomotive force therefor, a moving system having a coil arranged to lie in an air-gap in said field structure, vacuum tube power amplifier means for supplying said coil with audiofrequency energy, a power supply source having a certain amount of power available for said vacuum tube amplifier and said eld winding, means for supplying the plate circuit of said ampiiiler with substantially half of saidv available power, and means' for supplying said field winding'with the remaining part of said available power, thereby torender the acoustic output of said loudspeaker substantially a maximum.

. 2. In a radio receiver or the like, a plurality of vacuum tube amplifiers, a vacuum tube power output stage, a loudspeaker having a voice coil and a magnetic ield structure, a eld winding associated with said eld structure, a power source having a limited maximum power output,

connections between said power source and said vacuum tube ampliers for supplying said amplifiers with a part of the maximum output of said power source, connections forv supplying substantially half of the remainder of said maximum output to said power output stage, and connections for supplying the other half of said reminder to said ield winding, thereby to render the acoustic output of said loudspeaker substantially a maximum. v

3. In an audio frequency output system, an electrodynamic loudspeaker having a field structure, a field winding, and a moving system, class A. vacuum tube amplifier means `for supplying said moving system with energy, and a power supply means for supplying power to said eld winding and to the plate circuit of said ampliner means, characterized in that the power supplied 4to said plate circuit is substantially equal to the power supplied to said iield winding, thereby to render the acoustic voutput of said loudspeaker to-said power output stage, and connections for,

supplying the other half of said remainder to said eld winding, thereby to render the acoustic output, of said loudspeaker substantially a maximum.

. 5. Inan audio frequency output system, an electrodynamic loudspeaker having a field structure and a neldwinding wound on said structure, amovingcoil arrangedtolieinanairgapinsaid ileld structure, a vacuum tube-power amplier for supplylnl said moving coil with power at an audio frequency rate, a power supply source, said source having only a limited amount4 of power available for said power ampliner and said field winding, a path including said eld winding for supplying substantially halt ot* said available power to the plate circuit of said power amplifier, said ield winding having such a resistance that the other half of said limited available powerV is dissipated in said field winding and structure, thereby to render the acoustic output of said loudspeaker substantially a maximum.

6. In'an audio frequency output system, a series circuit including a loudspeaker field winding and the plate circuit lof a class A vacuum tube power output stage, and means for supplying said series circuit with electrical power, said means having only a limited available power output, said system being characterized in that said limited available power is divided substantially equally between said field winding and said plate circuit, thereby to render the acoustic output of said loudspeaker substantially a maximum.

'7. In an audio frequency output system, a circuit including in parallel a loudspeaker field ,winding and the plate circuit of a vacuum tube power output stage, said output stage supplying audio power to a moving system of said loudspeaker, means for -supplying said parallell circuit with electrical power, said means having only a limited available power output, said output system being characterized in that said available power is divided substantially equally between said eld winding and said vacuum tube plate circuit whereby the maximum eillciency' is oba tained from said system. Y

8. In an audio frequency outputsystem', a circuit including in parallel a loudspeaker iield winding and the plate circuit of a vacuum tube power output stage, said output stage being of the class A type and adapted to supply audio power to the moving system of said loudspeaker,

means for supplying said parallel circuit with electrical power, said means having only a limited available power output, said output system being characterized in that said available power is divided substantially equally between said field winding and said vacuum tube plate circuit whereby the maximum emciency is obtained from said system.

9. In an audio frequency output system, an electrodynamic loudspeaker having a ileld structure, a iield winding, and a moving system, vacuum tube amplifier means for supplying said moving system with energy, and a power supply means for supplying power to said eld winding and to the plate circuit of said ampliiler means, characterized in-that the-ratio of the power supplied the anode circuit of the output stage to the power supplied'said tleldwinding is not less than 0.4 nor greater than 2.5.

B5 10. In a radio receiver or the like, a plurality of vacuum tube ampliilers, a vacuum tube 'power output stage, a loudspeaker having. a voice coil and a magnetic field structure. a field winding associated with said iield structure, al Power source having a certain rated power output, circuits for supplying said amplifiers with a portion of said rated output, circuits for supplying said power loutput-stage with another portion of said rated output, and circuits for supplying said ileld winding with the remainder of said rated output, said receiver being characterized in that the ratio 'of the power supplied the anode circuit of the output stage to the powerl supplied said iield winding is not less than 0.4 nor greater than 2.5. 

